SELF DIAGNOSTIC CIRCUIT FOR CONTACT INPUT CIRCUIT

摘要

PROBLEM TO BE SOLVED: To improve the operation rate and reliability by adding a function for detecting degradation of reduction of luminance of a light emitting portion 4a of a photocoupler 4 to a self diagnostic circuit for a contact input 1 using a photocoupler 4 and by enabling change of a board of the contact input circuit 1 before the photocoupler 4 has a failure. SOLUTION: In this self diagnostic circuit for the contact input circuit, a degradation diagnostic energizing circuit 18 (or failure, degradation diagnostic energizing circuit 24) for applying a diagnostic current through a current limiting resistance 20 (26) is provided in parallel with external contacts 3 for carrying diagnostic current to the photocoupler 4 by limiting such that an output of a received light of the degraded photocoupler 4 does not reach an on-decision- level of a detection circuit 12 through the current limiting resistance 20 (26) and degradation of input photocouplers 4 is determined by the outputs of the detection circuits 12.